There is considerable interest in the non-destructive evaluation of the integrity of the mechanical bond between two structures. This interest has become particularly acute in the field of integrated circuit manufacturing. In the construction of an integrated circuit (IC) package, a small die, of silicon or other semiconductor material, is formed having the desired circuits printed thereon. The planar die is then bonded to a larger substrate such as a header, carrier or lead frame. The bond must have good mechanical strength and must provide a high level of thermal contact such that heat generated in the die during use is conducted to the substrate to prevent the die from overheating.
The necessity of creating a high quality bond is particularly important in the new, state of the art IC packages, where the dies are relatively large and generate more heat than ever before. Unfortunately, the bonding techniques now known do not always insure a high quality interconnection. The techniques used today frequently cause stresses to be generated in the silicon die which lead to an unsatisfactory bond. For example, one method of bonding includes the use of epoxy resins. When an epoxy resin cures, it shrinks or contracts causing stresses to build-up resulting in a poor quality bond. Another method employed is a solder technique carried out at a relatively higher temperature. However, as the components cool after the solder-bonding, differential contraction occurs also giving rise to stresses. Accordingly, since the bonding methods in use today are less than satisfactory, there is a great need for a means to evaluate integrity of the the bond achieved between the die and the substrate.
In the prior art, a few methods have been developed for evaluating the quality of the bond. For example, in die shear testing, the die is subjected to a mechanical force for examining the shear strength of the bond. This test, although direct, is generally destructive and thus not practical for automated large scale production. Other methods known in the prior art include x-ray radiography, infra-red microscopy, and transient thermal testing. Although the latter techniques are all essentially non-destructive, each has severe limitations related to applicability over a wide range of devices, reliability and possible automation.
Accordingly, it is an object of the subject invention to provide a new and improved method for non-destructively evaluating the quality of the bond between two members.
It is a further object of the subject invention to provide a new and improved method for assessing the integrity of the bond between two members which utilizes thermoacoustic microscopy.
It is another object of the subject invention to provide a new and improved method for evaluating the quality of the bond between two members by analyzing the plate-mode resonant signature of the combination.
It is still a further object of the subject invention to provide a new and improved method particularly suited for assessing the integrity of the bond achieved between an integrated circuit die and a substrate.
It is still another object of the subject invention to provide a new and improved method for obtaining the plate-mode resonant signatures of an integrated circuit die and substrate combination utilizing thermo-acoustic microscopy in order to evaluate the integrity of the bond achieved between the members.